Holography is a display technology in which a viewer sees a scene that varies as the angle at which the viewer views the scene changes. An example of a conventional hologram that is now in common use is the small image that is often imbedded in the front surface of a credit card. In that usage, the hologram is provided as a security or anti-counterfeiting, device, because of the difficulty involved in producing a hologram.
A great deal of information is required to describe a visually complicated three-dimensional scene over a reasonable angle of view. In practical holography systems, one has only a limited amount of hologram bandwidth in which to encapsulate this information. Therefore, techniques for holographic stereogram computation seek to sample the scene minimally, and encode this information holographically using some efficient scheme (often involving a table of precomputed diffractive patterns). Conventional stereogram computation techniques employ a fixed capture and projection geometry, and fixed spatial and parallax resolutions for any three-dimensional scene to be captured and processed for display. However, a three-dimensional image that is distorted, has aberrations, or is otherwise unconvincing is obtained if one fails to adjust various parameters such as the capture and image plane positions, capture and projection spatial resolution, and number of views used to sample the scene (parallax resolution).
FIG. 1 is a schematic flow diagram of the computation process that is performed in preparing a conventional computed stereogram of the prior art. A realistic three-dimensional image of a three-dimensional object or scene (generally referred to as a “target”) is to be displayed to a viewer of the projected holographic stereogram. In overview, the holographic stereogram is created by capturing (or synthesizing) a number of horizontal parallax only (HPO) views of the three-dimensional object, the number of views given by an integer n, as shown in box 102. One requirement imposed in conventional computed stereogram technology is matching of the rendering and projection geometries. The next step, represented by box 104, is computation of one basis fringe for each view direction, so that n basis fringes are computed. The computed stereogram is assembled as indicated in box 106, in an iterative process wherein a pixel vector p having components pi is retrieved by taking a slice through the image volume. The vector components pi are used to scale each basis fringe bi, and the resulting sum is assigned to a hologram element (called a holopixel) h, as shown in equation (1). In the notation used here, vectors are represented by boldface symbols.                     h        =                              ∑            i            M                    ⁢                                           ⁢                                    p              i                        ⁢                          b              i                                                          (        1        )            Finally, the holopixels are normalized to fit the available bit depth of the digital store or frame buffer.
Computational holography techniques have followed one of two general directions. A first method, fully-computed holograms, involves interference modeling of scenes populated by holographic primitives. A second approach, conventional computed holographic stereograms, involves parallax displays optically similar to holographic stereograms. In the fully-computed technique, the holographic pattern depends on the scene geometry. The three-dimensional geometry itself is populated with primitives such as spherical or cylindrical emitters. The emitter interference with a reference wave is computed to generate the hologram. A hologram computed in this fashion exhibits accurate reconstruction throughout the scene volume. However, its underlying diffractive structure is not useful to project any other scene geometry. The full computation must be done afresh every time changes in the scene texture, shading, or geometry occur. In the interest of efficiency, algorithms have been described to make this recomputation less burdensome, by using table lookup for instance, or by making incremental changes to the hologram.
In computational holography the conventional stereogram technique offers a fixed holographic “screen” through which any scene geometry is multiplexed, i.e., a multiple number of views of the scene, each from a different direction, are superimposed. Accordingly, once fixed, the conventional stereogram technique lacks configurability. Conventional stereograms typically affix the projection plane to the hologram plane and use a precomputed set of basis fringes to multiplex parallax views. The holopixels are arrayed on the hologram plane in an abutting fashion. This technique tends to introduce artifacts into the diffracted field from the basis fringes themselves, and the phase discontinuities between them.